The following Commonly Owned Patent and Patent Applications describe novel methods for producing a self-supporting ceramic body by oxidation of a parent to form a polycrystalline material of an oxidation reaction product and, optionally, metallic constituents:
(A) U.S. Pat. No. 4,713,360, which issued on Dec. 15, 1987 from U.S. application Ser. No. 818,943, filed Jan. 15, 1986, which is a continuation-in-part of Ser. No. 776,964, filed Sept. 17, 1985, which is a continuation-in-part of Ser. No. 705,787 filed Feb. 26, 1985, which is a continuation-in-part of U.S. application Ser. No. 591,392 filed Mar. 1 6, 1984, all in the name of Marc S. Newkirk et al and entitled "Novel Ceramic Materials and Methods for Making the Same"; and
(B) Ser. No. 822,999, filed Jan. 27, 1986, which is a continuation-in-part of Ser. No. 776,965, filed Sept. 17, 1985, which is a continuation-in-part of Ser. No. 747,788, filed June 25, 1985, which is a continuation-in-part of Ser. No. 632,636, filled July 20, 1984, all in the name of Marc S. Newkirk et al and entitled "Methods of Making Self-Supporting Ceramic Materials"; and
(C) Ser. No. 819,397, filed Jan. 17, 1986, which is a continuation-in-part of Ser. No. 697,876, filed Feb. 4, 1985, both in the name of Marc S. Newkirk et al. and entitled "Composite Ceramic Articles and Methods of Making Same."
The entire disclosures of each of the aforesaid Commonly Owned Patent Applications and Patent are incorporated herein by reference.
As explained in these Commonly Owned Patent Applications, novel polycrystalline ceramic materials or polycrystalline ceramic composite materials are produced by the oxidation reaction between a parent metal and a vapor-phase oxidant, i.e. a vaporized or normally gaseous material, as an oixidizing atmosphere. The method is disclosed generically in the aforesaid Commonly Owned Patent . In accordance with this generic process, a parent metal, e.g. aluminum, is heated to an elevated temperature above its melting point but below the melting point of the oxidation reaction product to form a body of molten parent metal which reacts upon contact with a vapor-phase oxidant to form the oxidation reaction product. At this temperature, the oxidation reaction product, or at least a portion thereof, is in contact with and extends between the body of molten parent metal and the oxidant, and molten metal is drawn or transported through the formed oxidation reaction product and towards the oxidant. The transported molten metal forms additional oxidation reaction product upon contact with the oxidant, at the surface of previously formed oxidation reaction product. As the process continues, additional metal is transported through this formation of polycrystalline oxidation reaction product thereby continually "growing" a ceramic structure of interconnected crystallites. The resulting ceramic body may contain metallic constituents, such as non-oxidized constituents of the parent metal, and/or voids. In the case of an oxide as the oxidation reaction product, oxygen or gas mixtures containing oxygen (including air) are suitable oxidants, with air usually being preferred for obvious reasons of economy. However, oxidation is used in its broad sense in all of the Commonly Owned Patent Applications and patent and in this application, and refers to the loss or sharing of electrons by a metal to an oxidant which may be one or more elements and/or compounds.
In certain cases, the parent metal may require the presence of one or more dopants in order to favorably influence or facilitate growth of the oxidation reaction product, and the dopants are provided as alloying constituents of the parent metal. For example, in the case of aluminum as the parent metal and air as the oxidant, dopants such as magnesium and silicon, to name but two of a larger class of dopant materials, are alloyed with aluminum and utilized as the parent metal. The resulting oxidation reaction product comprises alumina, typically alpha-alumina.
The aforesaid Commonly Owned Patent Applications (B) disclose a further development based on the discovery that appropriate growth conditions as described above, for parent metals requiring dopants, can be induced by applying one or more dopant materials to the surface or surfaces of the parent metal, thus avoiding the necessity of alloying the parent metal with dopant materials, e.g. metals such as magnesium, zinc and silicon, in the case where aluminum is the parent metal and air is the oxidant. With this improvement, it is feasible to use commercially available metals and alloys which otherwise would not contain or have appropriately doped compositions. This discovery is advantageous also in that ceramic growth can be achieved in one or more selected areas of the parent metal's surface rather than indiscriminately, thereby making the process more efficiently applied, for example, by doping only one surface, or only portion(s) of a surface, of a parent metal.
Thus, the aforesaid Commonly Owned Patent Applications describe the production of alumina as an oxidation reaction product readily "grown" to relatively large sizes, which then can be a useful source for alumina products. The present invention provides a method for obtaining substantially pure alumina derived from alumina produced by the aforementioned oxidation reaction process wherein, e.g., aluminum parent metal was reacted with an oxygen-containing vapor-phase oxidant.
Novel ceramic composite structures and methods of making them are disclosed and claimed in the aforesaid Commonly Owned Patent Applications (C) which utilize the oxidation reaction to produce ceramic composite structures comprising a substantially inert filler infiltrated by the polycrystalline ceramic matrix. A parent metal positioned adjacent to a mass of permeable filler is heated to form a body of molten parent metal which is reacted with a vapor-phase oxidant, as described above, to form an oxidation reaction product. As the oxidation reaction product grows and infiltrates the adjacent filler material, molten parent metal is drawn through previously formed oxidation reaction product into the mass of filler and reacts with the oxidant to form additional oxidation reaction product at the surface of the previously formed product, as described above. The resulting growth of oxidation reaction product infiltrates or embeds the filler and results in the formation of a ceramic composite structure of a polycrystalline ceramic matrix embedding the filler. For example, in the case of employing doped aluminum as the parent metal, air as the oxidant, and alumina particles or powder as the permeable filler, a composite is formed as described above which typically consists essentially of alumina particles in an alumina matrix having various metallic constituents dispersed therethrough.
In a further aspect of the present invention, it has been discovered that in the process of growing an alumina oxidation reaction product as a matrix through an alumina filler material, that when relatively impure forms of alumina fillers are employed, particularly those containing silicates, they react with the parent aluminum metal during the process to yield purer alumina and reduced metallic constituents, such as silicon. The process therefore can provide a source of high purity alumina from a lower purity alumina source.